I apologize if this information is already available as I do not have a lot of time to search around this forum.

I currently have one BES 4.1.3 implementation with 2 exchange 2003 servers working with the BES. Is it possible to enable users on an exchange 5.5 server to work with our BES? All of this is under one AD domain.

Heh, you were asking correctly; everyone apparently went on a vacation ... Yes you can indeed do this as long as the BES Admin account has access to the 5.5 server and the users you're looking to add are in the GAL.

One of the questions which I get asked when people are looking at mixed mode environments is confusion over CDO versions. Of course the CDO.dll versions for Exchange 5.5 will be different to those for Exchange 2003, and some people aren't sure which version to go with on their BES. In these scenarios you want to go for the most recent CDO file (Exchange 2003) as this should be backwards compatible with your Exchange 5.5 setup.

One question. The 5.5 people only exist on our current GAL (2003 exchange) by way of a contact pointing to their SMTP address. From the BES, I can select add user and see that contact although it only references the SMTP- not an exchange mailbox.

I have to admit I've never used BES with Exchange 5.5 ... BES uses the GAL to pull associated information on the location of the users mailbox. If it is just an SMTP address I don't see how it would be able to connect to the mailbox.

Hopefully someone with more experience in Exchange 5.5 / BES can help out here.

I think you'll find that the 5.5 server and the 2003 server must be "joined" in the same org/site. In other words, the GAL must reflect the mailboxes from both servers.

My original BES was against a 5.5 installation and in the 5.5 domain (there was a trust between the 5.5 domain and AD domain). When I migrated to 2003, I joined the 5.5 site (and turned on the ADC) and the Site Rep Service kept the two directories in sync. Anyway, when I migrated a 5.5 mailbox to the 2003 server, BES "migrated" right along with it. Basically, since both servers were in the same "site", BES was able to find out which server had the mailboxes. Later, I "moved" the BES to the AD domain and shut down 5.5.